Blue color filter element

ABSTRACT

A blue color filter having a blue filter layer comprising a first pigment having a maximum absorption at a wavelength within the range of from 550 to 650 nm, and a second pigment having a maximum absorption at a shorter wavelength than the first pigment within the range of from 500 to 600 nm, wherein the blue filter layer has chromaticity coordinates (x, y) in the 1931 CIE XYZ calorimetric system, calculated using CIE Standard Illuminant D65, that satisfy the expressions 0.134≦x≦0.15 and 0.03≦y≦0.06.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned U.S. patent application Ser. No.11/393,767 filed Mar. 30, 2006, entitled “Efficient white Light OLEDDisplay With Filters”, by Hatwar et al., the disclosure of which isincorporated herein by reference. This application is part of a seriesof four applications filed concurrently under attorney dockets 93114,93116, 93153, and 93154.

FIELD OF THE INVENTION

The present invention relates to color filters for electronic displays.

BACKGROUND OF THE INVENTION

In recent years, it has become necessary that image display devices havehigh-resolution and high picture quality, and it is desirable for suchimage display devices to have low power consumption and be thin,lightweight, and visible from wide angles. With such requirements,display devices (displays) have been developed where thin-film activeelements (thin-film transistors, also referred to as TFTs) are formed ona glass substrate, with display elements (for example, organiclight-emitting diode layers to produce light, or liquid-crystal layersto block light from a backlight) then being formed on top.

A problem with displays combining white-emitting devices with colorfilters is that the combination of emitter and color filters mustprovide a good color gamut for the reproduction of a wide range ofcolors. Color filters used in this way must have good spectroscopiccharacteristics, with sufficient transmittance with the predeterminedvisible light region and no unnecessary transmittance in other regionsof the visible spectrum. This has been difficult to achieve. The bluefilter of a Sony Qualia television suffers from significanttransmittance in the green region of the spectrum, which is notdesirable for high blue color purity.

Much work has been done to identify good color filters and color filtercombinations for liquid crystal displays (LCD), e.g. “Liquid CrystalDisplays”, Ernst Leudner ed., John Wiley & Sons (2001), pp. 28-296;“High Performance Pigments”, Hugh M. Smith, John Wiley & Sons, pp.264-265; Kudo et al., Jpn. J Appl. Phys., 37 (1998), pp. 3594-3603; Kudoet al., J. Photopolymer Sci. Tech. 9 (1996), pp. 109-120; Sugiura, J. ofthe SID, 1(3) (1993), pp. 341-346; FU et al., SPIE, Vol. 3560, pp.116-121; Ueda et al., U.S. Pat. No. 6,770,405; and Machiguchi et al.U.S. Pat. Nos. 6,713,227 and 6,733,934.

Despite such improvements, display color reproduction has remained fullof compromises. For example, the standards for color television gamut,as described by Fink in “Color Television Standards”, McGraw-Hill, NewYork (1955), and in Recommendation ITU-R BT.709-5, “Parameter values forthe HDTV standards for production and international programme exchange”,have seldom been met. The former NTSC reference describes a good redprimary as having 1931 CIE x,y chromaticity coordinates of x=0.67 andy=0.33, while a good green primary has coordinates of x=0.21 and y=0.71.The latter HDTV reference defines a good blue primary as the originalPAL/SECAM blue having coordinates of x=0.15 and y=0.06. Commerciallyavailable televisions fall short of these standards and have acompromised color gamut. Takizawa, in US 2004/0105265, teaches a redfilter that can achieve an x value as high as 0.65 and a y value as highas 0.33, which falls short of the NTSC reference red primary in x.Yamashita, in U.S. Pat. No. 6,856,364, teaches a red filter that canachieve an x value as high as 0.665 and a y in the range from 0.31 to0.35. While this is an improvement over Takizawa, a red primary thatmeets or exceeds the x value of the NTSC primary would result in a purerred color. Yamashita further teaches a blue filter wherein the x valuecan range from 0.13 to 0.15 and the y value can only be as low as 0.08,and a green filter wherein the x value can range from 0.22 to 0.34 witha y value ranging from 0.56 to 0.65. Both of these fall short of therespective desired primary x,y values, which if achieved would result inpurer blue and green colors, respectively.

Additionally, liquid crystal displays commonly available often use abacklight such as a cold-cathode fluorescent light (CCFL). It is acharacteristic of CCFL sources commonly available that, while itprovides white light consisting of a variety of wavelengths of thevisible spectrum, the light is often more intense in a few narrow bandsof the spectrum. These bands are generally centered in the red, thegreen, and the blue regions of the spectrum. The color filters neededwith such light sources do not need to be especially narrow to provide agood color gamut. For example, a red filter can allow a transmission“tail” into parts of the green region of the spectrum, so long as thetail region does not include the major green emission peak, and stillprovide good color with such a light source.

Organic light-emitting diodes (OLEDs) provide another light source fordisplays. Unlike LCDs, which have a single full-display light source,OLED displays only produce light at the pixels that are required to bebright at a given time. Therefore, it is possible for OLED devices toprovide displays that have reduced power requirements under normalusage. There has been much interest in broadband-emitting OLED devicesin color displays. Each pixel of such a display is coupled with a colorfilter element as part of a color filter array (CFA) to achieve apixilated multicolor display. The broadband-emitting structure is commonto all pixels, and the final color as perceived by the viewer isdictated by that pixel's corresponding color filter element. Therefore,a multicolor or RGB device can be produced without requiring anypatterning of the emitting structure. An example of a white CFAtop-emitting device is shown in U.S. Pat. No. 6,392,340. Kido et al., inScience, 267, 1332 (1995) and in Applied Physics Letters, 64, 815(1994), Littman et al. in U.S. Pat. No. 5,405,709, and Deshpande et al.,in Applied Physics Letters, 75, 888 (1999), report white light-producingOLED devices. Other examples of white light producing OLED devices havebeen reported in U.S. Pat. No. 5,683,823 and JP 07,142,169.

One property of broadband OLED displays is that, while they can varysomewhat in emission intensity at different wavelengths, they generallydo not have the intense peaks characteristic of CCFL sources. Therefore,common color filters that provide adequate color gamut when coupled witha CCFL display may not provide good results with OLED displays. Theexample above of a red color filter with a “tail” into a portion of thegreen region of the spectrum can provide an adequate red emission for aCCFL source, but be totally unsuitable for use with an OLED device.

Therefore, it is a problem to be solved to produce color filters thatcan be coupled with broadband OLED devices to provide displays withimproved color rendition.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a bluecolor filter to provide improved color rendition, particularly with abroadband-emitting OLED device; a display containing the filter; and amethod of making the filter. Improved color rendition includes improvedcolor gamut and related properties such as improved 1931 CIE x,ychromaticity coordinates and improved spectral curve shape.

This object is achieved by a blue color filter having a blue filterlayer comprising:

a. a first pigment having a maximum absorption at a wavelength withinthe range of from 550 to 650 nm;

b. a second pigment having a maximum absorption at a shorter wavelengththan the first pigment within the range of from 500 to 600 nm; and

c. wherein the blue filter layer has chromaticity coordinates (x, y) inthe 1931 CIE XYZ colorimetric system, calculated using CIE StandardIlluminant D65, that satisfy the expressions 0.134≦x≦0.15 and0.03≦y≦0.06.

Advantages

It is an advantage of this invention that it can produce color displayswith an improved combination of color and efficiency relative toexisting displays. This invention can provide better color gamut andthus better rendition of colors. While existing color filters couldprovide better color gamut by using thicker filters, this invention canprovide the improved color gamut with reduced efficiency loss.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a to 1 d show example pixel configurations that can be used foran electronic display using this invention;

FIG. 2 a shows a cross-sectional view of one embodiment of an electronicdisplay that can be used with this invention; and

FIG. 2 b shows another embodiment of an electronic display that can beused with this invention.

Since device feature dimensions such as layer thicknesses are frequentlyin sub-micrometer ranges, the drawings are scaled for ease ofvisualization rather than dimensional accuracy.

DETAILED DESCRIPTION OF THE INVENTION

The term “electronic display” refers to a display wherein electronicentities control the intensity of the different areas of the display.Such electronic entities can include e.g. an off-panel driver and aseries of horizontal and vertical electrodes in a passive-matrixdisplay, or an array of thin-film transistors (TFTs) in an active-matrixdisplay. Such displays can include liquid crystal displays (LCDs) andorganic light-emitting-diode (OLED) displays. The term “OLED display”,“OLED device” or “organic light-emitting display” is used in itsart-recognized meaning of a display device comprising organiclight-emitting diodes as pixels. The term “multicolor” is employed todescribe a display panel that is capable of emitting light of adifferent hue in different areas. In particular, it is employed todescribe a display panel that is capable of displaying images ofdifferent colors. These areas are not necessarily contiguous. The term“full color” is commonly employed to describe multicolor display panelsthat are capable of emitting in at least the red, green, and blueregions of the visible spectrum and displaying images in any combinationof hues. The complete set of colors that can be generated by a givendisplay is commonly called the color gamut of the display. The red,green, and blue colors constitute the three primary colors from whichall other colors can be generated by appropriate mixing. However, theuse of additional colors to extend the color gamut or within the colorgamut of the device is possible. The term “hue” refers to the intensityprofile of light emission within the visible spectrum, with differenthues exhibiting visually discernible differences in color. The term“pixel” is employed in its art-recognized usage to designate an area ofa display panel that can be stimulated to emit light independently ofother areas. It is recognized that in full-color systems, several pixelsof different colors will be used together to generate a broad range ofcolors, and a viewer can term such a group a single pixel. For thepurposes of this discussion, such a group will be considered severaldifferent colored pixels.

The terms “maximum absorption” and “maximum transmittance” as usedherein refer to the maximum light absorption and maximum lighttransmission, respectively, of color filters and color filter layerswithin the visible portion of the spectrum, i.e. from 400 nm to 700 nm.Red color filters are color filters that have a maximum transmittancesubstantially in the range of 600 nm to 700 nm. Green color filters arecolor filters that have a maximum transmittance substantially in therange of 500 nm to 600 nm. Blue color filters are color filters thathave a maximum transmittance substantially in the range of 400 nm to 500nm.

FIG. 1 illustrates example pixel configurations that can be used for anelectronic display using this invention. FIG. 1 a shows a stripe patternconfiguration of a device with group of pixels 20 a. Group of pixels 20a includes red, green, and blue color-gamut-defining pixels 21 a, 21 b,and 21 c. FIG. 1 a is a common example of an RGB display. FIG. 1 b showsa configuration of a device with group of pixels 20 b including red,green, and blue color-gamut-defining pixels 21 a, 21 b, and 21 c as wellas extra pixel 21 d, which can be a within-gamut pixel (e.g. white) orcan be another color-gamut-defining pixel. One common arrangementutilizing FIG. 1 b is an RGBW display, wherein portions of the display,e.g. within-gamut pixel 21 d, would not have a color filter. FIG. 1 cshows another pattern configuration of a device with group of pixels 20c. FIG. 1 d shows another pattern configuration of a device with groupof pixels 20 d. Other patterns can also be applied to the presentinvention, including patterns with more than 4 pixels. While in theabove-mentioned examples, the pixels are shown to be arranged in acertain order, the pixels can be arranged in other embodiments havingdifferent orders, and other embodiments can have pixels with differingsizes and shapes.

There are numerous configurations of color filters and displays withwhich this invention can be practiced. Turning now to FIG. 2 a, there isshown a cross-sectional view of one embodiment of a bottom-emittingelectronic display 10 that can be used with this invention. Electronicdisplay 10 is an OLED device well known in the art. An organicelectroluminescent (EL) element 70, comprising hole-injecting layer 35,hole-transporting layer 40, light-emitting layers 45 and 50,electron-transporting layer 55, and electron-injecting layer 60, isprovided over an OLED substrate 80. Current is provided by cathode 90and anodes 30 a, 30 b, and 30 c. The display includes at least threeseparate filters, e.g. red color filter 25 a, green color filter 25 b,and blue color filter 25 c, each of which is a separate emitting unitwith its own anode 30 a, 30 b, and 30 c, respectively.

Color filters are often provided on a substrate. In FIG. 2 a, thesubstrate is also the device substrate 20. Turning now to FIG. 2 b,there is shown another embodiment of an electronic display with colorfilters. Electronic display 15 is a top-emitting device. Color filters25 a, 25 b, and 25 c have been provided on a separate color filtersubstrate 85 that is placed over the electronic display after theelectronic and emissive layers are provided. It will be understood thatother arrangements of color filters commonly known in the art can beused with this invention. Further, other embodiments of electronicdisplays can be used, e.g. tandem OLED devices, liquid crystal displays,etc.

Color Filter Pigment Preparation

The milling that has been used in the art for color filter pigmentscommonly produces material with a wide range of particle sizes up to 500nm. It has been found that by milling the pigment particles to a narrowparticle size range, where the particle size is predominantly less than100 nm, yields improved color filter properties. A method for producingparticles of this type has been taught by Santilli et al. in U.S. Pat.No. 5,738,716, and by Czekai et al. in U.S. Pat. No. 5,500,331, thecontents of which are incorporated herein by reference. This method willbe referred to herein as micromedia milling.

The process of preparing color filters from pigments commonly involvesthree steps: (a) a dispersing or milling step to break up the pigment toa dispersion of the primary particle; (b) a dilution and/or mixing stepin which the dispersed pigment concentrate is diluted with a carrier andother addenda, which can include other pigment dispersions, to acoating-strength pigment dispersion; and (c) coating a color filterlayer from the coating-strength pigment dispersion onto a substrate.Step (a) can be further detailed as: (a1) providing a pigment mixturecontaining a pigment and a carrier for the pigment, and optionally adispersant; (a2) mixing the pigment mixture with milling media; (a3)introducing the mixture into a high-speed mill; (a4) milling the mixtureto obtain a pigment dispersion wherein the pigment particles have thedesired size; and (a5) separating the dispersion from the milling media.

In the milling step, the pigment is usually suspended in a carrier(typically the same carrier as that in the coating-strength slurry)along with rigid, inert milling media. Mechanical energy is supplied tothis pigment dispersion, and the collisions between the milling mediaand the pigment cause the pigment to deaggregate into its primaryparticles. A dispersant or stabilizer, or both, is commonly added to thepigment dispersion to facilitate the deaggregation of the raw pigment,to maintain colloidal particle stability, and to retard particlereaggregation and settling.

There are many different types of materials which can be used as millingmedia, such as glasses, ceramics, metals, and plastics. In a usefulembodiment, the grinding media can comprise particles, preferablysubstantially spherical in shape, e.g., beads, consisting essentially ofa polymeric resin. Desirably the beads have sizes in the range of 10 to100 microns, as described by Czekai et al.

In general, polymeric resins suitable for use as milling media arechemically and physically inert, substantially free of metals, solvent,and monomers, and of sufficient hardness and friability to enable themto avoid being chipped or crushed during milling. Suitable polymericresins include crosslinked polystyrenes, such as polystyrene crosslinkedwith divinylbenzene, styrene copolymers, polyacrylates such aspoly(methyl methylacrylate), polycarbonates, polyacetals, such asDerlin™, vinyl chloride polymers and copolymers, polyurethanes,polyamides, poly(tetrafluoroethylenes), e.g., Teflon™, and otherfluoropolymers, high density polyethylenes, polypropylenes, celluloseethers and esters such as cellulose acetate, poly(hydroxyethylmethacrylate), poly(hydroxyethyl acrylate), silicone containing polymerssuch as polysiloxanes and the like. The polymer can be biodegradable.Exemplary biodegradable polymers include polylactides, polyglycolids,copolymers of lactides and glycolide, polyanhydrides, poly(iminocarbonates), poly(N-acylhydroxyproline) esters, poly(N-palmitoylhydroxyprolino) esters, ethylene-vinyl acetate copolymers,poly(orthoesters), poly(caprolactones), and poly(phosphazenes). Thepolymeric resin can have a density from 0.9 to 3.0 g/cm³. Higher densityresins are especially useful inasmuch as it is believed that theseprovide more efficient particle size reduction. Especially useful arecrosslinked or uncrosslinked polymeric media based on styrene.

Milling can take place in any suitable grinding mill. Suitable millsinclude an airjet mill, a roller mill, a ball mill, an attritor mill, avibratory mill, a planetary mill, a sand mill, and a bead mill. A highspeed mill is particularly useful. By high speed mill we mean millingdevices capable of accelerating milling media to velocities greater thanabout 5 meters per second. The mill can contain a rotating shaft withone or more impellers. In such a mill the velocity imparted to the mediais approximately equal to the peripheral velocity of the impeller, whichis the product of the impeller revolutions per minute, π, and theimpeller diameter. Sufficient milling media velocity is achieved, forexample, in Cowles-type saw tooth impeller having a diameter of 40 mmwhen operated at 9,000 rpm. Useful proportions of the milling media, thepigment, the liquid dispersion medium and dispersant can vary withinwide limits and depends, for example, upon the particular materialselected and the size and density of the milling media etc. The processcan be carried out in a continuous or batch mode.

In batch milling, a slurry of<100 μm milling media, liquid, pigment, anddispersant is prepared using simple mixing. This slurry can be milled inconventional high energy batch milling processes such as high speedattritor mills, vibratory mills, ball mills, etc. This slurry is milledfor a predetermined length of time to allow comminution of the activematerial to a minimum particle size. After milling is complete, thedispersion of active material is separated from the milling media by asimple sieving or filtration with a barrier to the milling media but notthe milled pigment, e.g. a filter with a pore size of 5 μm.

In continuous media recirculation milling, a slurry of<100 μm millingmedia, liquid, pigment, and dispersant can be continuously recirculatedfrom a holding vessel through a conventional media mill which has amedia separator screen adjusted to>100 μm to allow free passage of themedia throughout the circuit. After milling is complete, the dispersionof active material is separated from the milling media by simple sievingor filtration.

With either of the above modes the useful amounts and ratios of theingredients of the mill grind will vary widely depending upon thespecific materials. The contents of the milling mixture comprise themill grind and the milling media. The mill grind comprises pigment,dispersant and a liquid carrier such as water. For aqueous filterslurries, the pigment is usually present in the mill grind at 1 to 50weight %, excluding the milling media. The weight ratio of pigment todispersant is 20:1 to 1:2. The high speed mill is a high agitationdevice, such as those manufactured by Morehouse-Cowles, Hockmeyer et al.

The dispersant is another important ingredient in the mill grind. Usefuldispersants include sulfates (e.g. sodium dodecyl sulfate), sulfonates(e.g. N-methyl-N-oleoyl taurate), acrylic and styrene-acrylic copolymerssuch as those disclosed in U.S. Pat. Nos. 5,085,698 and 5,172,133 (e.g.Joncryl 678), and sulfonated polyesters and styrenics such as thosedisclosed in U.S. Pat. No. 4,597,794. Other patents referred to above inconnection with pigment availability also disclose a wide variety ofuseful dispersants. The dispersants used in the examples are potassiumN-methyl-N-oleoyl taurate (KOMT) and Joncryl 678.

The milling time can vary widely and depends upon the pigment,mechanical means and residence conditions selected, the initial anddesired final particle size, etc. For aqueous mill grinds using theuseful pigments, dispersants, and milling media described above, millingtimes will typically range from 1 to 100 hours. The milled pigmentconcentrate is conveniently separated from the milling media byfiltration.

The carrier for the pigment can be an aqueous carrier medium or anon-aqueous solvent. Useful solvents have been disclosed by Czekai etal., and also in U.S. Pat. No. 5,145,684, U.S. Pat. No. 5,679,138, andEP 498,492, the disclosures of which are incorporated herein byreference. The aqueous carrier medium is water, an aqueous saltsolution, or an aqueous solvent mixture comprising water and at leastone water-miscible co-solvent. Selection of a suitable mixture dependson requirements of the specific application, such as desired surfacetension and viscosity, the selected pigment, drying time of the colorfilter layer, and the type of material onto which the pigment dispersionwill be coated. Representative examples of water-miscible co-solventsthat can be selected include (1) alcohols, such as methyl alcohol, ethylalcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butylalcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, andtetrahydrofurfuryl alcohol; (2) ketones or ketoalcohols such as acetone,methyl ethyl ketone, and diacetone alcohol; (3) ethers, such astetrahydrofuran and dioxane; (4) esters, such as ethyl acetate, ethyllactate, ethylene carbonate, and propylene carbonate; (5) polyhydricalcohols, such as ethylene glycol, diethylene glycol, triethyleneglycol, propylene glycol, tetraethylene glycol, polyethylene glycol,glycerol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, and thioglycol;(6) lower alkyl mono-or di-ethers derived from alkylene glycols, such asethylene glycol mono-methyl (or -ethyl) ether, diethylene glycolmono-methyl (or -ethyl) ether, propylene glycol mono-methyl (or -ethyl)ether, triethylene glycol mono-methyl (or -ethyl) ether, and diethyleneglycol di-methyl (or -ethyl) ether; (7) nitrogen containing cycliccompounds, such as pyrrolidone, N-methyl-2-pyrrolidone, and1,3-dimethyl-2-imidazolidinone; and (8) sulfur-containing compounds suchas dimethyl sulfoxide and tetramethylene sulfone.

Useful non-aqueous solvents include hydrocarbons, alcohols, polyols,ethers, and esters. Solvents known to be useful for this process includetoluene, hexane, ethanol, butanol, glycol, and PGMEA.

This treatment results in pigment particles wherein at least 90 weightpercent of the particles have a particle size less than 300 nm. Often,100% of the particles have a particle size less than 300 nm, andconveniently less than 200 nm. It is suitable that 100% of the particleshave a particle size less than 100 nm; however, this is not possible inall cases, and it is useful that at least 90 volume percent of thepigment particles have a particle size less than 100 nm, and desirablyless than 50 nm. In some cases, 90 volume percent of the pigmentparticles can have a particle size less than 30 nm. Usefully, no morethan 10 volume percent of the pigment particles have a particles sizeless than 5 nm.

Coating-Strength Dispersion Preparation

In general it is desirable to make the pigment dispersion in the form ofa concentrated mill grind, which is subsequently diluted to theappropriate concentration and further processed if necessary for use incoating. This technique permits preparation of a greater quantity ofpigment slurry from the equipment. If the mill grind was made in asolvent, it can be diluted with water and/or optionally other solventsto the appropriate concentration. If it was made in water, it can bediluted with either additional water or water-miscible solvents to thedesired concentration. If the color filter requires a mixture ofpigments, it is useful at this point to mix pigment dispersions thathave been milled separately. By dilution and/or mixing, the pigmentdispersion is adjusted to the desired viscosity, color, hue, saturationdensity, and area coverage for the particular application.

In the case of organic pigments, the coating dispersion can contain upto approximately 30% pigment by weight, but will generally be in therange of approximately 0.1 to 20%, and conveniently approximately 5 to15%, by weight of the total dispersion composition for most color filtercoating applications. If an inorganic pigment is selected, thedispersion will tend to contain higher weight percentages of pigmentthan with comparable dispersions employing organic pigments, and can beas high as approximately 75% in come cases, since inorganic pigmentsgenerally have higher specific gravities than organic pigments.

The amount of aqueous carrier medium is in the range of approximately 70to 98 weight %, and conveniently approximately 80 to 95 weight %, basedon the total weight of the dispersion. A mixture of water and apolyhydric alcohol, such as diethylene glycol, is useful as the aqueouscarrier medium. In the case of a mixture of water and diethylene glycol,the carrier medium usually contains from about 30% water/70% diethyleneglycol to about 95% water/5% diethylene glycol. Useful ratios areapproximately 60% water/40% diethylene glycol to about 95% water/5%diethylene glycol. Percentages are based on the total weight of thecarrier medium.

It can be desirable to add additional dispersant to the mixture. Usefuldispersants have been described above.

The ability to coat a given surface can be affected by the surfacetension of the coating-strength dispersion. Control of surface tensionsis accomplished by additions of small amounts of surfactants. The levelof surfactants to be used can be determined through simple trial anderror experiments. Anionic, nonionic, and cationic surfactants can beselected from those disclosed in U.S. Pat. Nos. 5,324,349; 4,156,616 and5,279,654 as well as many other surfactants. Commercial surfactantsinclude the Surfynols® from Air Products; the Zonyls® from DuPont andthe Fluorads® from 3M. A useful surfactant for these dispersions isSurfactant 10G from Dixie Chemical.

Coating of Pigments

To form color filters, pigments are often coated onto a substrate. Forexample, a color filter layer including the pigments can be coated ontoany of a variety of rigid and non-rigid transparent or semi-transparentmaterials, such as glass or plastic. The substrate can be a substrateused solely for forming a color filter, which can be attached to adisplay device. In another embodiment, the substrate can have other usesas well. For example, a color filter layer or array of color filterlayers can be coated onto the bottom of a bottom-emitting display devicesubstrate. In yet another useful embodiment, the pigments can be coatedover the top of an emitting layer that forms part of a display device.The display device can be an electronic display, such as an LCD displayor an OLED display.

Any of a variety of well-known coating and printing techniques can beused to prepare a color filter from the coating-strength pigmentdispersion. These techniques can include, but are not limited to,extrusion-type hopper (X-hopper) coating, spin coating, spray coating,ultrasonic spray coating, knife coating, and gravure coating. Thedispersion can be aqueous or non-aqueous. The coated dispersion is thentypically allowed to dry to form a solid or semi-solid coating.Alternatively, the slurry can include e.g. gelling materials orcrosslinking monomers to produce a solid or semi-solid coating. Thecoating-strength pigment dispersion can include one or more photoresistcompounds well-known in the art as useful for patterning color filters,e.g. in an array of colored pixels for an electronic display. In such acase, processing of the coated dispersion can include patterned exposureand post-exposure processing to form a patterned color filter.

The final color filter layers desirably comprise at least 10% colorpigment, conveniently at least 25% color pigment, and usefully at least50% color pigment by weight.

Blue Filter Pigments

A useful blue color filter according to this invention has good lighttransmittance in the blue region of the spectrum and good lightabsorption in the red and green regions of the spectrum. One usefulembodiment of this blue color filter has a first pigment having goodtransmittance in the blue region and a maximum absorption at awavelength within the range of from 550 to 650 nm, and a second pigmenthaving good transmittance in the blue region and a maximum absorption ata wavelength within the range from 500 to 600 nm. A useful first pigmentis a metallophthalocyanine pigment, and is conveniently copperphthalocyanine or a copper phthalocyanine derivative. A useful secondpigment is a dioxazine pigment, and is conveniently Pigment Violet 23.

When pigment particles are prepared as described herein, useful ratiosof the first pigment to the second pigment have been found to be in therange of 20:80 to 95:5 by weight. A color filter layer prepared fromsuch dispersions can have good transmittance in the blue region of thevisible spectrum while having good absorption in other regions of thespectrum. The blue filter layer so prepared can have a maximumtransmittance of 60% or more at a wavelength of 450 nm, but of no morethan 10% at all wavelengths in the range from 515 nm to 700 nm, andusefully no more than 5% at all wavelength in the range from 520 nm to700 nm. Such a filter layer has chromaticity coordinates (x,y) in the1931 CIE XYZ colorimetric system, when calculated using CIE StandardIlluminant D65 or Standard Illuminant C, that satisfy the expressions0.134≦x≦0.15 and 0.03≦y≦0.06, provided that such combination ofcoordinates is within the spectrum locus defined by the 1931 CIE x,ychromaticity diagram. As will be seen, this is a very pure blue color.

Dispersion Preparation Preparation of Pigment Violet 23 dispersion.

A mixture of Pigment Violet 23 was prepared by adding 39 g PigmentViolet 23 Cromophtal Violet GM (Ciba Specialty Chemicals), 11.7 gpotassium N-oleyl-N-methyltaurate dispersant, 249.3 g high purity waterand 360 g 50-micron milling media composed of crosslinked polystyrenedivinyl benzene to a 2L stainless steel vessel jacketed with chilledwater. The mixture was stirred with a 50 mm diameter high-shear Cowlesdisperser blade at an average rate of 3800 rpm for 48 hours. Aftermilling, the dispersion was separated from the milling media byfiltration through a 5 micron fiberglass filter and further diluted withhigh purity water to a concentration of 8.62 wt % pigment.

Preparation of Pigment Blue 15:6 Dispersion.

A mixture of Pigment Blue 15:6 was prepared by adding 39 g PB15:6Heliogen Blue D6700T (BASF), 11.7 g potassium N-oleyl-N-methyltauratedispersant, 249.3 g high purity water and 360 g 50-micron milling mediacomposed of crosslinked polystyrene divinyl benzene to a 2L stainlesssteel vessel jacketed with chilled water. The mixture was stirred with a60 mm diameter high-shear Cowles disperser blade at an average rate of2700 rpm for 23 hours. After milling, the dispersion was separated fromthe milling media by filtration through a 5 micron fiberglass filter andfurther diluted with high purity water to a concentration of 9.98 wt %pigment.

Table 1 shows the relative amounts of pigment, dispersant, and water inthe dispersions prepared above that were used to make color filters asdescribed below. Table 1 also shows particle size distribution of thepigments in the dispersions, as measured by dynamic light scatteringusing a Microtrace UPA150 particle analyzer. Examination of thedispersion by transmitted light microscopy at 1110× magnification showedall particles to be well dispersed.

TABLE 1 Pigment Pigment Pigment Violet 23 Blue 15:6 dispersant KOMT KOMTwt % pigment 8.62 9.98 wt % dispersant 2.59 2.99 wt % water 88.79 87.03100% by volume 0.1022 0.1719 less than (microns): 90% by volume less0.0248 0.0304 than (microns): 50% by volume less 0.0116 0.0126 than(microns): 10% by volume less 0.0093 0.0100 than (microns):

Filter Preparation Inventive Blue Filter (B_(i))

50 g of the above Pigment Blue 15:6 dispersion was mixed with 18.5 g ofthe above Pigment Violet 23 dispersion, 68.5 g Joncryl 678, and 10 dropsof a 10% Surfactant 10 G solution. The resulting slurry was then coatedonto a polyester sheet using an x-hopper syringe coater at a rate of 1.4cm³/ft². This provided a coating with an average thickness when dry of2.6 microns.

First Comparative Filters

Comparative Blue Filter 1 (B_(c1)) was obtained from a commerciallyavailable LCD television.

The visible transmittance spectra of the above filters were measuredwith a Perkin-Elmer Lambda 12 spectrometer with an integrating filter.The results are shown in the table below.

TABLE 2 Color Filter: Inventive Blue Filter Comparative Blue Filter 1Peak Transmittance 448 nm 462 nm % T at Peak 66% 76.5%   Transmittance %T at 430 nm 57% 61% % T at 480 nm 43% 70% % T at 505 nm 10% 43% % T at512 nm 5% 33% % T at 517 nm 3% 25% Green & Blue <1.5% from 526 to <1.5%from 552 to Transmittance 700 nm 700 nm

The inventive blue filter peak transmittance is at a wavelength that ishypsochromic to the comparative blue filter peak, which means that ithas less unwanted transmittance into the green portion of the visiblespectrum. Also the transmittance of the bathochromic tail of theinventive blue filter is much lower at wavelengths from 505 to 700 nm,and especially in the near-green region of 500 to 550 nm, than that ofthe comparative blue filter. These two spectral transmittancecharacteristics mean that the inventive blue is a purer blue (i.e. lesscyan) than the comparative blue filter, due to the small particle sizeof the pigments in the inventive filter. A further example of this canbe seen when compared to the blue filter of a Sony Qualia television,which contains similar materials with unknown formulation. When measuredas described herein, it has a transmittance at 515 nm of 17%, which ismuch greater than the inventive filter at the same wavelength, and isnot desirable for good blue color purity.

The color purity of the filters can be further demonstrated withexcitation purity, which is a common CIE metric used to measure thepurity of a color point plotted on the 1931 CIE chromaticity diagram.The spectral characteristics of selected illuminants can be cascadedwith the spectral transmittance of the color filters, and with the 1931CIE color matching functions, as described in “Colorimetry”, CIEPublication 15:2004 3rd Edition, published by the CIE Central Bureau inVienna, Austria. The result of this cascade is a set of chromaticitycoordinates that pertains to a given illuminant on the 1931 CIEchromaticity diagram. The excitation purity is the length of the linesegment joining the illuminant point with the color point relative tothe length of the line segment joining the illuminant point, the colorpoint, and the spectrum locus point. A color with an excitation purityof 1.0 lies on the spectrum locus and represents the purest spectralcolor possible. One can further calculate a difference from thepredetermined standard by the formula:

Delta CIE x,y=SQRT[(x₁−x_(NTSC))²+(y₁−y_(NTSC))²]

When the blue filters are cascaded with CIE Standard Illuminant D65, andthe 1931 CIE color matching functions, Table 3 shows the resultingchromaticity coordinates.

TABLE 3 Blue Filter Filter or Primary: Rec. Comparative 709 BlueInventive Blue Blue Primary 1931 CIE x, y (Std. III. C) 0.1452, 0.05010.1353, 0.1019 0.15, 0.06 Excitation Purity (C) 0.9610 0.8964 0.9265Delta CIE x, y from HDTV 0.0110 0.0444 0 Rec. 709 Primary (C) 1931 CIEx, y 0.1445, 0.0523 0.1341, 0.1096 0.15, 0.06 (Std. III. D65) ExcitationPurity (D65) 0.9533 0.8900 0.9209 Delta CIE x, y from HDTV 0.0095 0.05210 Rec. 709 Primary (D65)

The resulting CIE chromaticity coordinates for the inventive blue filterare higher in x and much lower in y than the comparative blue filter,making the inventive filter blue less cyan and hence a more pure bluecolor. The inventive blue has a higher excitation purity than thecomparative blue filter, and it is close to the excitation purity forthe HDTV Rec. 709 blue primary, as defined in Recommendation ITU-RBT.709-5, “Parameter values for the HDTV standards for production andinternational programme exchange”. This ITU recommendation defines an“aim” blue primary that is purer than all preceding standards. Theinventive blue chromaticity lies close to the HDTV Rec. 709 blue primarychromaticity so that its x,y delta is very almost zero. The x,y deltafor the comparative blue filter is more than five times larger, makingthe comparative blue filter a much less pure blue color. The inventiveblue filter is a much purer blue than either the comparative blue filteror the blue filter taught by Yamashita in U.S. Pat. No. 6,856,364.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

The patents and other publications referred to herein are incorporatedherein by reference.

PARTS LIST

-   10 electronic display-   15 electronic display-   20 a group of pixels-   20 b group of pixels-   20 c group of pixels-   20 d group of pixels-   21 a pixel-   21 b pixel-   21 c pixel-   21 d pixel-   25 a red color filter-   25 b green color filter-   25 c blue color filter-   30 a anode-   30 b anode-   30 c anode-   35 hole-injecting layer-   40 hole-transporting layer-   45 light-emitting layer-   50 light-emitting layer-   55 electron-transporting layer-   60 electron-injecting layer-   70 organic EL element-   80 OLED substrate-   85 filter substrate-   90 cathode

1. A blue color filter having a blue filter layer comprising: a. a firstpigment having a maximum absorption at a wavelength within the range offrom 550 to 650 nm; b. a second pigment having a maximum absorption at ashorter wavelength than the first pigment within the range of from 500to 600 nm; and c. wherein the blue filter layer has chromaticitycoordinates (x, y) in the 1931 CIE XYZ colorimetric system, calculatedusing CIE Standard Illuminant D65, that satisfy the expressions0.134≦x≦0.15 and 0.03≦y≦0.06.
 2. The blue color filter of claim 1wherein at least 90 volume percent of the first pigment particles have aparticle size less than 100 nm and at least 90 volume percent of thesecond pigment particles have a particle size less than 100 nm.
 3. Theblue color filter of claim 2 wherein no more than 10 volume percent ofthe first pigment particles have a particle size less than 5 nm and nomore than 10 volume percent of the second pigment particles have aparticle size less than 5 nm.
 4. The blue color filter of claim 3wherein at least 90 volume percent of the first pigment particles have aparticle size less than 50 nm and at least 90 volume percent of thesecond pigment particles have a particle size less than 50 nm.
 5. A bluecolor filter having a blue filter layer comprising: a. a first pigmenthaving a maximum absorption at a wavelength within the range of from 550to 650 nm wherein at least 90 volume percent of the first pigmentparticles have a particle size less than 300 nm; b. a second pigmenthaving a maximum absorption at a shorter wavelength than the firstpigment within the range of from 500 to 600 nm wherein at least 90volume percent of the second pigment particles have a particle size lessthan 300 nm; and c. the blue filter layer has a transmittance of 60% ormore at a wavelength of 450 nm and of no more than 10% at allwavelengths in the range from 515 nm to 700 nm.
 6. The blue color filterof claim 5 wherein the blue filter layer has a transmittance of no morethan 5% at all wavelengths in the range from 520 nm to 700 nm.
 7. Theblue color filter of claim 5 wherein 100% of the first pigment particleshave a particle size less than 200 nm and 100% of the second pigmentparticles have a particle size less than 200 nm.
 8. The blue colorfilter of claim 7 wherein at least 90 volume percent of the firstpigment particles have a particle size less than 100 nm and at least 90volume percent of the second pigment particles have a particle size lessthan 100 nm.
 9. The blue color filter of claim 8 wherein no more than 10volume percent of the first pigment particles have a particle size lessthan 5 nm and no more than 10 volume percent of the second pigmentparticles have a particle size less than 5 nm.
 10. The blue color filterof claim 9 wherein at least 90 volume percent of the first pigmentparticles have a particle size less than 50 nm and at least 90 volumepercent of the second pigment particles have a particle size less than50 nm.
 11. The blue color filter of claim 5 wherein the first pigment isa metallophthalocyanine pigment.
 12. The blue color filter of claim 11wherein the first pigment is a copper phthalocyanine pigment.
 13. Theblue color filter of claim 12 wherein the weight ratio of the firstpigment to the second pigment is in the range of 20:80 to 95:5.
 14. Theblue color filter of claim 13 wherein the weight ratio of the firstpigment to the second pigment is about 72:28.
 15. The blue color filterof claim 5 wherein the second pigment is a dioxazine pigment.
 16. Theblue color filter of claim 15 wherein the second pigment is PigmentViolet
 23. 17. The blue color filter of claim 5 wherein the pigmentparticles are produced by micromedia milling.
 18. A display devicecomprising the blue color filter of claim
 5. 19. The display device ofclaim 18 wherein the display device is an electronic display.
 20. Thedisplay device of claim 19 wherein the electronic display is an OLEDdisplay.
 21. The blue color filter of claim 5 wherein the blue filterlayer has chromaticity coordinates (x, y) in the 1931 CIE XYZcolorimetric system, calculated using CIE Standard Illuminant D65, thatsatisfy the expressions 0.134≦x≦0.15 and 0.03≦y≦0.06.
 22. A method offorming a blue color filter having a blue filter layer comprising thesteps of: a) providing a pigment mixture containing a first pigment, acarrier for the pigment, and a dispersant; b) mixing the pigment mixturewith rigid milling media having an average particle size less than 100micrometers; c) introducing the mixture of step b) into a high-speedmill; d) milling the mixture until a pigment dispersion is obtainedwherein at least 90 volume percent of the pigment particles have aparticle size less than 100 nm; e) separating the milling media from thedispersion milled in step d); f) repeating steps a) to e) for a secondpigment; g) combining the dispersions of first and second pigments; andh) coating the resulting dispersion onto a substrate to form a bluefilter layer.
 23. The method of claim 22 wherein the carrier for thepigment is water or an aqueous solvent mixture.
 24. The method of claim22 wherein the carrier for the pigment is a non-aqueous solvent.